The Zenit-3SL rocket was carrying the Intelsat-27 (IS-27) satellite, said RKK Energia head Vitaly Lopota.

"There was a malfunction -- an emergency shutdown of the first-stage motor -- around 50 seconds into the flight," he said.

The Sea Launch consortium led by Energia launched the rocket from the floating platform Odyssey at an equatorial launch site in international waters in the Pacific Ocean at 10.56 a.m. Moscow time.

The rocket fell into the sea not far from the platform, which was not damaged in the failed launch.

The Zenit-3SL integrated launch vehicle is a liquid-propellant rocket consisting of three stages and a payload unit.

The first stage is powered by the RD-171M engine designed by Russia's NPO Energomash rocket engine design bureau.

The RD-171M is one of the most powerful rocket engines in the world, producing 740,000 kg of thrust at lift-off.

Sea Launch AG was formed in 1995 as a consortium of four companies from Norway, Russia, Ukraine and the US, and managed by US aerospace giant Boeing.

The project aimed to use a floating launch site to place rockets on the equator, the best possible location for launch, which gives the rocket additional speed on lift-off thanks to centripetal force caused by the earth's rotation.

That in turn means higher possible payloads for a given power of the launcher.

Sea Launch resumed operations in 2011 after a 30-month hiatus that saw passage through bankruptcy, a change of ownership from Boeing to Energia, and a move of the company headquarters from California to Switzerland.

Of the 34 previous Sea Launch rocket launches since 1999, two have failed, with a third placing its payload into an incorrect, but recoverable orbit.

The first failure occurred in March 2000, when a software error resulted in premature cut-off of the second stage, leaving its ICO F-1 satellite unable to reach orbit.

In June 2004, during the launch of an Apstar 5 satellite, the upper stage shut down 54 seconds early due to a wiring fault, leaving the satellite in a lower than planned orbit.

A Zenit-3SL exploded on the launch pad in January 2007, after an engine failure caused by debris in the turbo pump.